High speed optical scanning systems such as precision plotters, printers and the like are well known in the art. These devices are direct imaging systems and are used to fabricate printed circuit boards (PCB) and printing plates by raster scanning an exposure beam across a resist coated printed circuit board or plate which is further processed into a printed circuit board layer or a printing plate, respectively. A typical system as marketed by the Gerber Scientific Instrument Company, the assignee of the present application, consists of a magnetic tape drive, hard disk, computer interactive graphics terminal, image processor and an optical table having a movable scanner. The system may also include such optics, media carriage and electronics as is necessary to directly transfer computer aided design (CAD) data to a (PCB) layer or transfer fonts, graphics and half-toned images into a printing plate.
In operation, the direct imaging system is configured to receive on the write platen a planar substrate of aluminum (in the case of graphic arts) or copper clad dielectric (in the case of printed circuit boards) which has had an optically sensitive photopolymer applied to its surface. The computer modulates the intensity of an optical beam usually provided by a laser to expose selected portions of the substrate. Typically, there is a second beam reference which is also scanned simultaneously with the exposure beam for generating a scanned timing signal to accurately control the position of the exposure (or write beam) on the substrate. A flat field scanning system is sometimes employed to focus the beams and to accomplish the simultaneous scanning of the beams across a reference mask and the substrate, respectively. Precision air bearings are often used to guide the write platen as the substrate is imaged.
In known laser raster scanning systems where the optical deflector is a scanning multifaceted mirror, there are typically three scanning modes. The first mode is where the illuminated facet is underfilled. That is, less than the entire facet is filled with the exposure beam. In the second mode, the exposure beam overfills the facet by extending beyond the borders of the illuminated facet. The third and most desirable of the scanning modes is where the facet is exactly illuminated by the exposure beam. In addition, scanners operating in this third mode also comprise a means for displacing illumination beams (both the exposure and reference beams) relative to the rotating polygon or pyramid to track the facet and improve scanning efficiency. Known scanning systems which are characterized by facet tracking do so in an active manner. These systems include an electromechanical or acousto-optical device for physically displacing the beam relative to the polygonal facet. The active device is controlled by means of a controller which must be operated in a closed loop manner. In turn, closed loop operation requires additional components, such as a detector to provide optical feedback to the controller for calibration and accurate operation.
It would be desirable to provide a laser scanning system characterized by a facet tracking apparatus that is passive requiring no electronics for operation and which could be operated in an open loop manner. The present invention is drawn towards such a scanning system.